The present invention relates to a method of manufacturing a liquid crystal display panel. More specifically, the present invention relates to a method of manufacturing a high-quality liquid crystal display panel, in which proper internal pressure of a cell is maintained by a One-Drop-Fill (ODF) method.
Usually, a process of manufacturing an active matrix color liquid crystal display device using thin-film transistors (TFTs) as switching elements includes an array step of forming a wiring pattern, the switching elements on a glass substrate, a cell step of forming a liquid crystal display panel by arranging spacers between the glass substrate and a glass substrate opposite thereto and sealing liquid crystal between the glass substrates, and a module step of performing attachment of a driver IC, and mounting of a backlight. Among these steps, in the substep of arranging the spacers in the cell step, columnar spacers are formed on one or both of the opposite substrates, and a cell gap (cell thickness) between the opposite glass substrates is constantly maintained.
In recent years, a One-Drop-Fill (ODF) method has received attention in a process of injecting the liquid crystal. This One-Drop-Fill (ODF) method is, for example, a method in which a specified amount of liquid crystal is dropped on a surface of the array substrate, which is surrounded by a main seal formed in a periphery of the array substrate, the array substrate and the opposite substrate are bonded together in vacuum, and the liquid crystal is sealed therebetween. Conventionally, a vacuum injection method has been widely used. In the vacuum injection method, the array substrate on which the TFTs are formed, and the opposite substrate opposite thereto, on which color filters (CFs) are formed, are bonded together with a sealing material interposed therebetween, and then the liquid crystal is sealed between the substrates. As compared with this vacuum injection method, the One-Drop-Fill method as described above has the potential to reduce the manufacturing cost of the liquid crystal display panel and to improve mass productivity thereof because the time required for injecting the liquid crystal can be shortened.
In the case of manufacturing the liquid crystal display panel by this One-Drop-Fill method, in order to obtain a proper cell gap, it is important to align the columnar spacers evenly and to drop a predetermined amount of the liquid crystal in accordance with height and hardness of the columnar spacers. When the amount of liquid crystal gets out of balance, for example, when the amount of liquid crystal is too large, a defect of uneven gravity occurs in the liquid crystal in a cell. Alternatively, when the amount of liquid crystal is too small, foaming at low temperature and unevenness in black display, which is inherent in IPS (In-Plane-Switching liquid crystal), occur in the liquid crystal in the cell. It is known that malfunctions caused by such defects occur.
There are a number of methods proposed by others to address these defects. Japanese Patent Laid-Open No. 2002-107740, the disclosure of which is incorporated by reference herein, provides a method of measuring the drop amount of liquid crystal and collecting the liquid crystal when the amount is improper.
Japanese Patent Laid-Open No. 2001-281678, the disclosure of which is incorporated by reference herein, provides a method of measuring the height of the columnar spacers in advance by a laser displacement meter.
Japanese Patent Laid-Open No. H6-235925 (published in 1994) and Japanese Patent Laid-Open No. 2002-156638, the disclosure of which is incorporated by reference herein, provides a method of discharging the excessive liquid crystal from an opening provided on a part of the seal after the end of the ODF process, thus obtaining proper internal pressure of a cell.
It has been possible to control the amount of liquid crystal in the cell with a precision of approximately ±0.5% due to improvements in manufacturing and inspection tools. Notwithstanding this, there are still deficiencies in the prior art. When consideration is made for changes of specific gravity and viscosity of the liquid crystal due to a temperature change, a coating thickness of a film surface of the substrate, it has been very difficult to accurately adjust the amount of liquid crystal and to maintain the internal pressure of the cell at proper pressure. From such a viewpoint, for example, the method of measuring the drop amount of liquid crystal and collecting the unrequired amount of liquid crystal, as described in Japanese Patent Laid-Open No. 2002-107740, is not realistic.
Moreover, the method of measuring the height of the columnar spacers, as described in Japanese Patent Laid-Open No. 2001-281678, has problems. First, it is difficult to detect transparent columnar spacers by means of white-light interferometry or the like. Second, it is difficult to measure columnar spacers having tilted portions, and to automate the measurement. Accordingly, a large number of points cannot be measured rapidly. Next, the method of discharging the excessive liquid crystal after the end of the ODF process requires apparatuses for pressurization and sealing. Even if the apparatuses are actually used, it is difficult to obtain the proper gap. Furthermore, there emerge problems regarding a tact time and quality of a sealing port.
Furthermore, the columnar spacers are usually prepared by a photolithography technology by using photosensitive resin, and accordingly, it is difficult to control the height of the columnar spacers within a range where variations are small (for example, within a range of plus or minus several percent). Moreover, it is difficult to measure the height of the columnar spacers for each substrate in terms of measurement precision, variations depending on individual products and a measurement frequency. Typically, only one piece per several tens of pieces are sampled, and only values obtained by measuring several points on each sampled surface are used.
Furthermore, it is under these circumstances that measurement of hardness (elasticity/plasticity) of the columnar spacers or a shape thereof, and adjustment of the amount of liquid crystal sealed between the substrates based on the measurement values of the hardness or shape are performed. It is extremely difficult to timely detect, in the cell step, malfunctions based on the causes as described above, which occur in the liquid crystal panel, and there is a possibility that the defects may occur after shipment of the product. Therefore, there is a need for improved methods to solve the above described deficiencies.